Asphalt is a well-known cementitious material containing predominantly bitumens which occur in nature as such, or are obtained as the residue in the refining of petroleum. Asphalt has been used for many years in road paving applications. Many miles of existing highways consist of asphalt in combination with various aggregates such as gravel, crushed stone and sand.
Despite the widespread use of asphalt in road paving applications, asphalt suffers from certain limitations due to corrosion, deterioration and disintegration. Some of these problems are caused by mechanical action due to contact of wheels of vehicles and other objects in passing over the surfaces of the asphalt pavements. Other causes are due to the inherent nature of asphalt compositions which can range from hard to soft. If the composition is hard and brittle, it resists distortions. However, it cracks and allows water to enter, and the formation of ice further breaks up the asphalt. If the composition is too soft, it is too flexible and distorts. It is also known that as the asphalt ages, volatile components evaporate, causing brittleness and a decrease in the stability of the road surface. A typical asphalt road surface begins to show signs of deterioration and disintegration about 4-5 years after application.
To extend the time when repaving must occur, various preventive maintenance techniques are practiced. The problem with these preventive maintenance programs is that they are costly in terms of material and personnel for any advantage gained and provide no strength to the road surface. Further, the application of a layer on top of a cracked and deteriorated surface results in an unstable condition with the new layer cracking essentially where the old surface was cracked, i.e., reflective cracking.
Cost is an important consideration with an asphalt or modified asphalt, as is the durability of the surfaces, and the frequency and nature of required preventive maintenance in response to constant weather and vehicular-induced damage. In many cases, however, the available manpower and budgets are not sufficient to assure consistent adherence to a regular maintenance schedule.
Extensive development has recently taken place in which, in road paving applications, the asphalt is modified with various additives such as fibers or polymers or elastomers, such as rubber and epoxy.
It has been reported that flex and durability have been substantially improved with respect to paving and sealing compositions by including reinforcing fiber materials, such as fiberglass, and other synthetic polymeric fibers, such as polyolefins. See, for example, U.S. Pat. No. 4,663,370 issued to Marvel; U.S. Pat. No. 4,492,781 issued to Duszak et al.; U.S. Pat. No. 4,412,864 issued to Kurashige et al.; U.S. Pat. No. 4,316,829 issued to Roberts; U.S. Pat. No. 3,505,260 issued to Woodruff; and U.S. Pat. No. 3,474,625 issued to Draper et al. A preferred synthetic fiber material for such purposes, has been found to be polypropylene fiber, because of a high degree of compatibility with asphalt. Such material has been used in amount of 0.25%-10% by weight fiber per weight of asphalt concrete depending upon intended use.
While improving flex and durability, addition of synthetic fiber material such as polypropylene adversely affects pumpability and workability characteristics of the asphalt composition. Fiber clumping occurs in the preparation of the composition and during conventional smoothing operations (i.e., rolling and compaction).
Another substance that has been added to asphalt compositions is rubber. Two general processes are known for making rubber asphalt--one is the so-called "wet process," the other the so-called "dry process." In the wet process, the rubber is admixed with the asphalt cement prior to the addition of any aggregate. The rubber chemically reacts with the asphalt cement to modify the asphalt; such asphalt is often referred to as "chemically-modified asphalt." See, for example, U.S. Pat. Nos. 4,394,482; 4,333,866; 4,332,705; 4,440,816; 4,485,144; and 4,528,241 issued to Uffner; and U.S. Pat. Nos. 4,273,685; 4,404,316; 4,251,586; and 4,175,978 issued to Marzocchi et al.
In the dry process, the rubber is used as another aggregate along side the traditional mineral aggregates, and is added into the aggregate prior to admixing with the asphalt cement. See, for example, U.S. Pat. No. 4,086,291 issued to Svensson; U.S. Pat. No. 4,548,962 issued to Landmark. To date, none of the rubber asphalt compositions, made by either process, has achieved any degree of commercial success and widespread use, due in part to the high cost of manufacture. The present inventor is unaware of any attempts to combine fiber and rubber, but Duszak et al., U.S. Pat. No. 4,492,781, state that combining rubber and fiber to form a fiber-containing rubber asphalt seems incompatible.
Thus, notwithstanding the many known practical problems for asphalts or even asphalts modified with certain additives, the art has not adequately responded to date with the introduction of an asphalt concrete composition that has a surface of greater durability and flex and that resists corrosion, deterioration, and cracking, which is applicable over a wide range of temperature conditions, particularly lower temperatures, which can be laid down and cured within a reasonably short period of time, and yet which requires less maintenance and has a cost competitive with conventional asphalt.